Make lib feature-aware and conditionally import Pyth

[mcintyre94]

This allows environments like Playground to know which dependencies are required without being aware of feature flags

We still use an optional feature + cargo feature flag for the dependency when running locally

The result is Seahorse 🤝 Pyth 🤝 Playground

Closes: #75

[ameliatastic]

Whoops, looks like a lot of conflicts came up when I merged #92. Shouldn't affect the tests too much, though.

I'll review this soon - can you add that this will close #75?

[mcintyre94]

Rebased + added that :)

[ameliatastic]

LGTM!